Tag Archives: deformation

The picture above shows a boulder of Hot Springs Sandstone with well-developed sigmoidal veins. Sigmoidal veins – sometimes called tension gashes – form in rock by shear stress. That’s stress that causes adjacent parts of a rock to slide past one another. In the above picture the yellow arrows indicate the approximate orientation of the stresses that were applied to this boulder to create the sigmoidal veins.

Sigmoidal veins, at their inception, are shaped like parallel lines that bulge toward the center and taper at the ends. They originate due to tension created between the two opposing forces acting on the rock. Essentially the rock tears to alleviate this tension. If the shearing continues long enough, these openings in the rock begin to rotate. The eventual shape, seen above, is like the letter S. The ends of each S point opposite of the direction of the force that created them. Therefore, sigmoidal veins can indicate the forces at work on bedrock when it was buried underground.

The veins pictured here are at the edge of a parking lot next to the Arlington Hotel in Hot Springs Arkansas. After they developed the veins were in filled with quartz. The Hot Springs Sandstone is a member of the Mississippian Stanley Formation.

Above are several images of the same rock sample: a highly deformed quartzose siltstone collected from the Womble Formation, Ouachita Mountains, Arkansas. The uppermost image shows a cut and polished surface. The green line that’s been added to the picture defines a fracture that split the sample after it was cut. Ordinarily, that would be a bitter turn of events but, in this case, it was a fortunate accident. The fracture provides a rare, multi-dimensional view inside a tightly folded rock (lower photo). Luckily, the fracture propagated across the bedding rather than breaking along a bed, which makes the beds of the fold appear to fan out like a deck of cards showing a lot of the detail of the structure.

Pictured above is Dardanelle Rock located on the south side of the Arkansas River between the towns of Dardanelle and Russellville. The white truck in the lower right corner shows the scale of this outcrop. It was designated a Natural Area by the Arkansas Natural Heritage Commission in 1976.

The Arkansas River Valley is north of the Ouachita Mountains and is characterized by gently folded sedimentary rock that was subject, to a lesser extent, to the stress that folded the Ouachita Mountains. The rock pictured here is the south limb of a broad syncline, or down-warped fold. The north limb is about two miles to the northeast. The bedrock dips to the north (toward the white truck), goes sub-surface beneath the Arkansas River, then reverses dip direction and rises back to the surface just southwest of Russellville. If you could see a cross-section of the folded rock, it would look like giant a smiley face with the middle of the smile underground and the corners sticking up in opposite directions, two miles apart.

This picture gives perspective to the colossal size of geologic features geologists study. Folds like this one, which can trap upward-migrating fluid, are sometimes rich oil and gas reservoirs.

This is an anticline exposed on Mc Leod Street, southwest of Hot Springs, Garland County, Arkansas. It’s not unique as, anticlines are common in the Ouachita’s and other mountain ranges throughout the world. Most often though, these structures are large scale and cover expanses of land that can’t be viewed from a human vantage point. When they do form on a scale that’s small enough for human observation, we typically don’t have the benefit of a freshly blasted exposure like this one.

In fact, many times geologists must infer that folds like this exist in places deep underground that no one has or will ever see. That’s why, if you see a geologist on the side of the road, taking something like this in, as in the picture above, just let him have his little moment. The exposure is of deep marine sedimentary deposits of the Stanley Formation.

Tight recumbent folds in a fresh exposure of Bigfork Chert, Ouachita Mountains Arkansas. Thin chert beds divided by siliceous shale beds are the basic lithology of this Ordovician age (540 to 490 Ma) formation. Complicated structures like those pictured below are characteristic of the rocks that form the Ouachita Mountains. The complex folds and faults resulted from plate tectonic forces that compressed the rock and caused many of the structures to rotate and overturn.

Signs that summer is almost over despite the continued warm temperatures include the silencing of cicada calls one at a time. A welcome turn of events!

More “crazy-bedding” in the upper Imo

This week was a long one because we worked in two different field areas. On Monday and Tuesday we were back on Tick Creek looking at the last two major eastern drainages. Found good Cane Hill/Imo contacts in each and a fairly consistent irregular-, channel-bedded sandstone unit with abundant soft-sediment deformation near the top of the Imo. It’s so irregular that I was calling it “crazy-bedded” before long.

“Crazy-bedding” in the upper Imo

Yet more “crazy-bedding” in the upper Imo

There was also a section in the middle of the Cane Hill that was so perfectly cut by its east/west joint that it formed a smooth wall on the north side of the creek. There was a parallel joint face on the south side, but it was not nearly as well exposed.

Danny views joint face “wall” in Cane Hill

Buttresses on Bear Creek

On Tuesday evening we joined Angela Chandler and Lea Nondorf, also of the Survey, on Bear Creek in Marshall to work on several other projects the rest of the week. One ongoing project is to try to resolve edge-matching issues whenever we get a chance. This involves gathering new data in boundary areas between quads that were mapped by different people or who used a different stratigraphy. This week we were also looking for localities in the Imo interval for an upcoming field trip that Angela, Erin Smart and I are leading this spring for the GSA (Geological Society of America) conference in Fayetteville. While looking at various road cuts, we also took new points on an area in the corner of four quads.

Wednesday and part of Thursday we looked at several possible field trip stops in the Imo, none of which seemed particularly suitable for one reason or another. Mostly this is because the Imo doesn’t tend display good outcrops in this area due to its shaley composition and its typically being covered by the flaggy sandstone of the Cane Hill above. We did visit the type section which is in Sulphur Springs Hollow to see if that could be used as a stop, but deemed the area too rugged unless a very small, sturdy group of geologists sign up. A type section is an area where a formation or rock unit is first described and studied in detail. In this case the type section was proposed then summarily abandoned, but there are those working hard to see it reinstated eventually.

Coalified wood prints in Imo sandstone at the type section

Crinoids in red, fossiliferous Imo limestone at the type section

Another project that we are working on for the National Park Service is a compilation of all the quad maps along the Buffalo National River. Our agency and the US Geological Survey have each done about half of the quads in that area. Most of the quads mapped by our agency did not include the higher terrace levels above the river–some as high as 200 feet! This week we were able to get points on a few more of these terraces on the Snowball quad.

Smart Bluff above Arnold Bend on the Buffalo River

Calcite fracture-fillings used as building stones in St. Joe

Quartz crystal encrusted Boone chert used as building stones in St. Joe

We looked in Arnold Bend Thursday afternoon, and differentiated several terrace levels there, then on the way back to town, we stopped at a roadcut on Hwy. 65 that Angela knew about where there are quartz crystals growing in fractured Boone limestone and chert. Danny had stopped at St. Joe on a previous field trip and so directed us there to see the quartz and calcite crystals encrusted on some of the old building stones in town. These stones were no doubt found nearby along the several mineralized fault zones in the area.

Early morning fog on Bear Creek

The next morning there was fog on Bear Creek and we headed up to Jamison Bluff to look for Plattin in the riverbed where it had been mapped previously. This is part of the 6 mile section of the Buffalo between Woolum and Margaret White Bluff that dries up during the summer. All we could find was St. Joe, so that part of the map remains as is.

Jamison Bluff along the Buffalo River

River crossing at Woolum on the Buffalo River

Looking down on Skull Bluff on the Buffalo River

We crossed the Buffalo at Woolum and walked along the top of Skull Bluff to the Nars Cemetery most of which was covered with a terrace deposit (along with almost impenetrable black locust, cedar and briars).

And of course, we couldn’t leave the area without a quick trip to “the Nars” itself for Danny and Lea to see for the first time.

“The Nars”. Buffalo River left, Richland valley right.

“The Nars” is an almost sheer rock wall in the Boone formed as an erosional remnant between the valleys of the Buffalo River and Richland Creek. Quite impressive as usual. After that we had to get back to Little Rock and “the real world”. See you next week.